Monte Carlo simulations have been conducted to investigate the phase behavior of 2:1 hard-core electrolyte models. Two distinct cases have been considered: In the first, both bivalent cations and monovalent anions are spherical. In the second, bivalent cations are modeled as rigid dimers composed of two tangent hard spheres, each carrying a positive charge at the center. Critical temperatures and densities have been calculated as a function of the size asymmetry between positive and negative ions. The simulated critical temperature and critical density are strongly influenced by size asymmetry and by the shape of the ions. Changes in the critical constants are traced back to ground-state energy configurations of small ionic clusters. The trends observed in simulations for the critical temperature and density as a function of size asymmetry are shown to contradict the predictions of available theoretical formalisms.